Lesson 2 Diffractometers & Phase Identification Nicola Döbelin RMS Foundation, Bettlach, Switzerland February 11 14, 2013, Riga, Latvia
Repetition: Generation of X-rays Kα 1 Target (Cu, Mo, Fe, Co,...) Intensity Kβ Kα 2 Vacuum e 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Wavelength (nm) Cu Graphite Monochromator Cu Radiation Ni filter Ni-filtered Cu Radiation CuKα 1/2 Radiation 2
Repetition: Powder Diffraction λ n λ = 2 d sin(θ) (120) θ θ 2θ d (100) (010) Powder sample 3
Repetition: Powder Diffractometer Diffraction Cones «Secondary Beams» X-ray tube Primary Beam Powder Sample X-ray Detector scanning X-ray intensity vs. 2θ angle 4
Analogue Cameras Debye-Scherrer Camera: Powder in Glass Capillary Diffraction pattern recorded on photographic film Various alternative setups: Gandolfi Guinier Straumanis Bradley Seemann-Bohlin Camera 2θ angle http://adias-uae.com 5
Digital Diffractometer Debye-Scherrer Configuration Transmission Geometry Glass Capillary Foil Fluid Cell Capillaries are ideal for: Light atoms (Polymers, Pharmaceuticals) Small amounts Hazardous materials Air-sensitive materials 6
Bragg-Brentano Diffractometer Bragg-Brentano Configuration Reflective Geometry Flat powder sample Reflective Geometry is ideal for: Absorbing materials (Ceramics, Metals) Thin films Texture analysis 7
Instruments Lab Instrument Monochromator Configuration RTU Rigaku Ultima+ Graphite Monochromator Bragg-Brentano (Reflection) RTU PanalyticalX Pert Ni-Filter Bragg-Brentano (Reflection) LU BrukerD8 Ni-Filter Bragg-Brentano (Reflection) UppsalaUni BrukerD8 Ni-Filter Bragg-Brentano (Reflection) RTU Salaspils Bruker D8 Energy-dispersive Detector Bragg-Brentano (Reflection) RMS (Uni Bern) PanalyticalX Pert Ni-Filter Bragg-Brentano (Reflection) RMS (Uni Bern) PanalyticalCubiX Graphite Monochromator Bragg-Brentano (Reflection) 8
Bragg-Brentano Diffractometer Detector Kβ filter Flat powder sample X-ray tube More optical elements are required to control the beam pattern. 9
Beam Divergence Angle of Divergence Point focus Line Focus 10
Beam Divergence Limiting vertical divergence with a «divergence slit» (DS) Limiting horizontal divergence with a «Collimator» Collimator for Line focus = «Soller Slits» (SS): Non-parallel rays are blocked by lamellae Beam Mask: Limits the beam width 11
Beam Divergence Divergence Slit Soller Slit Beam Masks 12
Soller Slits Poster Viewed through Soller Slit 13
Bragg-Brentano Parafocusing Diffractometer Typical Configuration (with Kβ filter) Focusing circle Receiving slit X-ray tube Divergence slit Soller slit Anti-Scatter slit Anti-Scatter slit Soller slit Detector Kβ Filter Beam mask Goniometer circle Sample 14
Bragg-Brentano Parafocusing Diffractometer Typical Configuration (with secondary monochromator) Secondary monochromator Soller slit Receiving slit Anti-Scatter slit Detector Modern instruments are modular. Configuration can be changed easily. Sample PANalytical: «PreFIX» Bruker: «SNAP-LOCK» 15
Example: PANalytical X Pert Pro MPD Soller Slits Beam Mask Programmable Anti-Scatter Slit Ni-Filter Tube Programmable Divergence Slit Anti-Scatter Slit Sample Stage «Spinner» Soller Slit Detector 16
Optimum Settings: Divergence Slit 20 2θ Use narrower divergence slit. 20 2θ Wide divergence slit: Beam spills over sample at low 2θ angles. 80 2θ 17
Optimum Settings: Divergence Slit Fixed divergence slit: 20 2θ 80 2θ Low incident angle: - Low penetration depth - Large illuminated area High incident angle: - Deep penetration depth - Small illuminated area Irradiated Volume is constant Constant intensity of diffraction pattern Variable divergence slit: 20 2θ Low incident angle: - Narrow divergence slit - Low penetration depth 80 2θ High incident angle: - Wide divergence slit - Deep penetration depth Irradiated Area is constant Higher diffracted intensity at high 2θ angle 18
Fixed vs. Variable Divergence Slit More than 2x higher intensity at 90 2θ with variable DS Intensity [a.u.] fixed 20 30 40 50 60 70 80 90 Diffraction Angle [ 2θ] variable 19
Optimum Settings: Divergence Slit Correct! Wrong! Wrong! Sample holder Reduce «irradiated length» of divergence slit Use a smaller Beam Mask Primary beam Irradiated area Sample surface Recommendation: - Set divergence slit to «variable» - Adjust «irradiated length» and beam mask for maximum illumination - But avoid beam spill-over! 20
Optimum Settings: Divergence Slit Using sample holders of various sizes? Match your Divergence Slit and Beam Mask! Or else: Waste of intensity or Beam spill-over 21
Variable Divergence Slit: Irradiated Length 3000 5mm 10mm 15mm 2500 Intensity [counts] 2000 1500 1000 500 0 30.2 30.3 30.4 30.5 30.6 30.7 Diffraction Angle [ 2θ] Soller Slits: 0.02 rad Beam Mask: 10mm 22
Variable Divergence Slit: Irradiated Length 100 5mm 10mm 15mm 80 Intensity [%] 60 40 20 0 30.2 30.3 30.4 30.5 30.6 30.7 Diffraction Angle [ 2θ] Soller Slits: 0.02 rad Beam Mask: 10mm 23
Beam Mask 3500 5mm 10mm 20mm 3000 2500 Intensity [counts] 2000 1500 1000 500 0 30.2 30.3 30.4 30.5 30.6 30.7 Diffraction Angle [ 2θ] Soller Slits: 0.02 rad Irradiated Length: 10mm 24
Beam Mask 100 5mm 10mm 20mm 80 Intensity [%] 60 40 20 0 30.2 30.3 30.4 30.5 30.6 30.7 Diffraction Angle [ 2θ] Soller Slits: 0.02 rad Irradiated Length: 10mm 25
Soller Slits 7000 0.02rad 0.04rad 6000 5000 Intensity [counts] 4000 3000 2000 1000 0 30.2 30.3 30.4 30.5 30.6 30.7 Diffraction Angle [ 2θ] Beam Mask: 10mm Irradiated Length: 10mm 26
Soller Slits 100 0.02rad 0.04rad 80 Intensity [%] 60 40 Affects resolution at low 2θ angles (effect is less pronounced at high 2θ angles) 20 0 30.2 30.3 30.4 30.5 30.6 30.7 Diffraction Angle [ 2θ] Beam Mask: 10mm Irradiated Length: 10mm 27
Receiving Slit Strongly affects the resolution. (Not all instruments have a receiving slit) Parrish, W. Advances in X-ray diffractometry of clay minerals. X-ray analysis papers (W. Parrish, ed.), pp. 105-129. Centrex, Eindhoven, The Netherlands, 1965. 28
Summary: Optical Elements Optical Element Effect Too Small Too Large Divergence Slit Soller Slit Anti-Scatter Slit Beam Mask ReceivingSlit Adjusts beam length on the sample Reduces peak asymmetry Reduces background signal Adjusts beam width on the sample Adjustspeakwidth/ resolution Loss of intensity Loss of intensity, Better resolution Loss of intensity Loss of intensity Loss of intensity Better resolution Beam spills over sample More asymmetry, Less resolution High background Beam spills over sample Loss of resolution Higher intensity Kβ Filter Reduces Kβ peaks - - Graphite Monochromator Eliminates Kβ peaks - - 29
Detectors Detector Type Point Detector (0D) Linear Detector (1D) Area Detector (2D) Detector s window Position-sensitive «bins» Receiving slit Receiving slit determines active height Example Key Features 30 Linear array of solid state detectors Scintillation counter SOL-XE X Celerator LynxEye D/teX Ultra SOL-XE: Energy dispersive Fast 2D array of solid state detectors PIXcel VÅNTEC Needs receiving slit! Can be set to «0D mode» 2D image of Debye rings Can be set to «1D» and «0D» mode
Instruments Lab Instrument Monochr. Detector RTU Rigaku Ultima+ Graphite 0D RTU Panalytical X Pert Ni-Filter 1D X Celerator LU Bruker D8 Ni-Filter 1D LynxEye Uppsala Uni Bruker D8 Ni-Filter 1D LynxEye RTU Salaspils Bruker D8 Energy-disp. Detector 0D SOL-XE RMS (Uni Bern) Panalytical X Pert Ni-Filter 1D X Celerator RMS (Uni Bern) Panalytical CubiX Graphite 0D 31
Phase Identification Intensity [counts] A crystal structure will generate a characteristic XRD pattern. Feature Origin 1000 800 600 400 200 Peak positions - Symmetry of the unit cell - Dimensions of the unit cell Relative peak intensities - Coordinates of atoms in unitcell - Speciesofatoms Absolute peak intensities - Abundance of phase - Primary beam intensity Peak width - Crystallite size - Stress/Strain in crystal lattice 0 10 20 30 40 50 60 Diffraction Angle [ 2θ] 32
Phase Identification 1000 1. Extract peak positions 800 Intensity [counts] 600 400 200 0 10 20 30 40 50 60 Diffraction Angle [ 2θ] 33
Phase Identification 1000 1. Extract peak positions Sample 2. Compare 800 with data base Intensity [counts] 600 400 Corundum 200 Fluorite 0 10 20 30 40 50 60 Diffraction Angle [ 2θ] 34
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Databases Databases containing powder diffraction data (line positions) Database Publisher # of Entries Data sets PDF-2 PDF-4+ PDF-4/Minerals PDF-4/Organics ICDD (http://www.icdd.com) ICDD (http://www.icdd.com) ICDD (http://www.icdd.com) ICDD (http://www.icdd.com) 250 182 All 328 660 Inorganic 39 410 Minerals (Subset of PDF-4+) 471 257 Organics Commercial COD COD http://www.crystallography.net 215 708 All (excl. biopolymers) Open Access (Received funding by Research Council of Lithuania (2010-2011)) 36
Programmes for Search / Match Programme Publisher Supported Databases* HighScore PANalytical PDF-2/4 COD EVA Search/Match Bruker PDF-2/4 PDXL2 Rigaku PDF-2 COD RayfleX GE PDF-2/4 Sleve ICDD PDF-2/4 Match! Crystal Impact PDF-2/4 COD CSM Oxford Cryosystems PDF-2/4 Jade MDI PDF-2/4 + many more (see http://www.ccp14.ac.uk/solution/search-match.htm) *incomprehensive 37
Search / Match: Restrictions By chemical Composition By Subfile 38
Summary: Phase Identification I - Phases are identified from XRD patterns by comparing peak positions with database entries - Search/Match software & database are required - Various commercial / open programmes and databases - Qualitative (sometimes semi-quantitative) results are obtained - Phase identification is independent of Rietveld refinement (must be done before) 39
Question I: Polytypes Is powder XRD the ideal tool to distinguish and identify the following phases? Phase Composition Space Group Calcite CaCO 3 R-3c Magnesite MgCO 3 R-3c Siderite FeCO 3 R-3c Structurally very similar (polytypes) They generate similar diffraction patterns XRD provides no direct information on Ca/Mg/Fe content Only changes in unit cell dimensions. 40
Question I: Polytypes Calcite CaCO 3 - Similar diffraction patterns (mostly peak shifts) - Some information on Mg/Ca/Fe contens from unit cell dimensions Magnesite MgCO 3 Solution: Combine XRD with chemical analysis (ICP, XRF, EDX, XPS ) Siderite FeCO 3 41
Question II: Polymorphs Is powder XRD the ideal tool to distinguish and identify the following phases? Phase Composition Space Group Calcite CaCO 3 R-3c Vaterite CaCO 3 P63/mmc Aragonite CaCO 3 Pnam Structurally different (polymorphs) Chemical analyses not able to distinguish (chem. identical) XRD can easily distinguish 42
Question II: Polymorphs Calcite CaCO 3 - Strongly different diffraction patterns. - Easily identified by XRD Aragonite CaCO 3 Vaterite CaCO 3 43
Summary: Phase identification II - XRD is mostly sensitive to structural differences - Only little information on chemical differences - Chemical analyses (XRF, ICP, EDX, ) provide complementary information - Sometimes additional chemical information can be very helpful for phase identification - For a comprehensive material characterization, combine XRD with chemical analysis 44
Typical Slit Configuration 45
X-ray Mirrors Collimators and Divergence Slits cut off intensity There are no lenses for X-rays (Index of refraction for all materials ~1) Bragg diffraction can be used to construct mirrors Single crystal with parabolic surface: All beams coming from the tube focus are in diffraction condition for Kα 1/2 Goebel Mirror Focusing Mirror Parallel Kα 1/2 beam Focused Kα 1/2 beam 46
Polycapillary Optics Latest generation: Polycapillary glass fiber optics Conserves most of the primary beam intensity 47